Transport trucks that haul goods over great distances in Europe, the Americas and other parts of the world are often referred to as long-haul trucks. In addition to a bed, the cabins of long-haul trucks are often configured to include microwaves, air conditioners, heaters, refrigerators, televisions, stereos and other electric appliances that require significant amounts of power. Long-haul trucks or big rigs will travel hundreds of miles in a day, over many days, often stopping only long enough to allow the driver to eat and take care of personal necessities and to rest and sleep, but when they do stop, many drivers want to use two or more of these appliances at the same time, such as an air conditioner, microwave, and television.
Such conveniences are very important to many drivers, and given the increasing shortage of long-haul truck drivers, truck fleet owners are seeking new ways to attract drivers by providing a more luxurious cab environment. Some trucking fleets have up to 100% turn over from year to year because the drivers are not satisfied with the life style provided by the fleet company. Yet, it costs a trucking company at least three thousand dollars to train new drivers, even if they have prior experience, so obviously, the quality of life of the driver is a key to success in the industry.
When a long-haul truck does need to stop, the driver might be able to do so at a truck stop, which is a specialized facility for providing fuel, maintenance, parking, and convenience services. At other times, the trucks will stop wherever they can safely do so, such as at roadsides, rest stops and fueling stations. Although some truck stops provide auxiliary power tethers to the trucks, most drivers prefer to stop when and where it proves to be most convenient, and to idle their main engines while stopped to provide power to the cabin of the truck. In the United States, a typical long-haul truck idles 2,500 hours per year, with the main engine consuming as much as 1.2 gallons per hour. As fuel prices increase, however, it is getting prohibitively expensive for drivers to idle the main engines for hours at a time. At a fuel cost of $3.00 per gallon, idling costs as much as $9,000 per year. Furthermore, many countries are instituting laws that make it illegal to idle a truck engine for extended periods of time to cut down on air pollution. Idling the engine for hours also decreases the amount of time between engine overhauls without increasing the productivity of the vehicle.
Lastly, the noise generated by a running main engine, even at idle speeds, is considerable both inside and outside of the truck, and the amount and intensity of vibrations generated inside the cabin of the truck can be considerable. Reducing the noise generated by a truck when stopped will improve the quality of life for the driver and passengers of the truck as well as those around the truck, making it more acceptable for the trucks to stop for longer periods of time in certain areas. Reducing the vibrations within the trucks improves the quality of life for those inside the truck. There are relatively few things truck owners can do to make life more enjoyable for truck operators once they are on the road and maintain profitable businesses, but improving the quality of the operator's rest periods is an important one.
Accordingly, a number of companies have begun to supply auxiliary power units (APUs) to provide climate control and 120-volt power, to cut back on fuel consumption and air pollution, to reduce operating hours on the main vehicle engine, and to improve driver comfort and quality of life when on the road. A typical APU consumes about 0.2-0.3 gallons per hour, with significantly lower annual maintenance costs, thereby saving drivers/truck owners more than $7,000 per year in fuel costs alone. In the European Union, where long-haul trucks only idle about 1,800 hours per year, but fuel costs much more per gallon, the idle cost savings alone are over $8,500 per year.
The APUs currently on the market, however, share certain features and disadvantages. For example, most APUs use small diesel engines for power, but depending on the size of those engines, they may only be able to provide a limited amount of DC power. Others are incapable of providing AC power because they do not include an AC power generator. Furthermore, many of the diesel engines powered APU's are noisy to operate, even with a muffler installed on the exhaust system, and generate a significant amount of vibration throughout the cabin, especially when the engine is mounted under the cabin or attached to the frame rail of the truck, where the vibrations can readily travel to the cabin.
In many cases, airborne noise, such as that produced by the cylinder exhaust ports of the engine, is propagated in and transmitted through the exhaust pipe wall and radiated as shell noise or transmitted to the vehicle through the engine mounts. Mufflers are typically utilized to reduce exhaust noise. A muffler typically includes a resonating chamber, which is designed to have opposite sound waves collide and cancel each other out. Internal combustion engines also typically include an air intake system for receiving air that is later mixed with fuel and combusted in the engine cylinders. Noise from the engine, however, also typically travels through the air intake system, through the engine, and back into the atmosphere. In certain smaller engines, especially where the exhaust is in close proximity to the air intake, such as in an APU, noise traveling from the engine through the air intake can be a significant noise source. This is particularly true when the engine is operated at higher speeds, as is necessary when a two or three cylinder APU is utilized to drive an AC power generator.
Air intake silencers have often been used to muffle, and therefore reduce, the level of sound energy generated by these types of engines. An air intake silencer can also attenuate the sound waves produced by the entering air. Noise reduction is often achieved by the provision of sound deadening baffles within the silencer housing. Silencers are often constructed as a combined air silencer/filter module. For instance, a filter membrane may be supported around the silencer housing which defines the annular inlet into the silencer so that the air flow is filtered as it passes radially into the silencer housing. However, it is also know that the introduction of a filter in the airflow, especially near the silencer intake can introduce noise into the engine as well.
While mufflers and intake silencers can reduce noise, which in turn can reduce some vibrations, noise reduction techniques are not typically focused on vibration reduction as well, which might exist in the absence of significant noise.